In recent years, an increase in the integration of semiconductors, reduction in the distance of the wires and miniaturization of printed wiring boards have been required in accordance with demands for the development of the functions, miniaturization and reduction of the thickness of electronic apparatuses. Semiconductor devices and electric components in chip form such as bare chips, capacitors in chip form and inductors in chip form are mounted on such a printed wiring board.
However, such electric components are mounted only on conductor circuits on the external layers of a printed wiring board, and therefore, the quantity of electric components which are mounted on the printed wiring board is limited while such electric components are placed so as to protrude from the external surfaces of the printed wiring board, which hinders the miniaturization of the printed wiring board. In addition, in the case where electric components are mounted on positions of the outermost layers of a wiring board, freedom in the wiring design is restricted.
These problems become more significant as a greater number of layers are included in a printed wiring board. That is to say, the greater the number of layers included in a printed wiring board, the greater the quantity of wiring, while electric components are mounted solely on the external layers of the printed wiring board resulting in the reduction of the quantity of electric components mounted on the printed wiring board relative to the quantity of wiring, and therefore, the miniaturization of a printed wiring board due to an increase in the number of layers is limited by the quantity of electric components mounted on the printed wiring board.
In addition, as disclosed in Japanese Unexamined Patent Publication H11(1999)-126978, manufacture of a multi-layered wiring board has been proposed wherein an air space is provided in an insulation layer so that semiconductor elements and the like are mounted in this air space, and furthermore, such insulation layers made of photosensitive resin and wired circuits are sequentially layered so as to form the wiring board according to the prior art. According to this technology, it becomes possible to mount electric components within insulation layers so that an increase in the quantity of mounted electric components, miniaturization of a wiring board and an increase in the freedom of wiring are achieved to some extent while air is sealed in a gap that has occurred between the inner surface of an air space and an electric component, causing a high risk of a crack in an insulation layer and a defect due to damage of or disconnection of the wire of an electric component to occur as a result of a thermal expansion of air within the air space in the case where the wiring board is subjected to stress due to heat, and in addition, the manufacturing process for such a wiring board becomes complicated because an air space must be created in an insulation layer in accordance with the dimensions or the quantity of the mounted electric components. In addition, insulation layers and conductor circuits are sequentially layered to form a multilayer board in the case of fabrication of a multilayer board, and therefore, the formation of an insulation layer and the formation of a conductor circuit need to be repeated alternately, requiring time and effort for very complicated procedure in the manufacture of the multilayer board. In addition, a heat treatment is required for the curing to form an insulation layer whenever one layer of an insulation layer or conductor circuit is formed, and therefore, each conductor circuit formed in each layer has a different thermal history leading to a different contraction ratio of each conductor circuit pattern, which requires correction. In addition, at the time of the formation of an insulation layer on a conductor circuit pattern, a conductor circuit is buried in an insulation layer as a result of the curing of an insulating resin that has been melted for the formation of the insulation layer wherein unevenness in the wiring board that occurs when a conductor circuit is buried in an insulation layer becomes greater as a greater number of layers are included in the wiring board. In some cases, the unevenness in the wiring board causes a failure in the formation of an insulation layer that covers unevenness of a conductor circuit in a manner where an extremely thin portion is formed in the insulation layer causing a high risk of lowering the reliability of the insulation.
Furthermore, connection structures such as pad on via and via on via can not be formed according to such a conventional manufacturing method, and a problem arises wherein there is a limit in the increase in the density of the conductive circuit, and there is a limit in the reduction of the board area, preventing the reduction in the distance of signal paths.
In addition, a component is inserted in an insulation layer, and thereby, the length of a via in this insulation layer becomes extremely large, causing a big problem in the reliability of the conductive resistance of the via.
As described above, several problems exist in the conventional manufacturing method concerning the reduction of unevenness of a circuit layer by efficiently burying components in the layer as well as concerning the securing of connection reliability, and the quantity of mounting when components are mounted.
The present invention is provided in view of the above described problems, and an object thereof is to provide a wiring board sheet with a high reliability, as well as its manufacturing method, that allows miniaturization of a wiring board where electric components are mounted within an insulation layer, and thereby, the quantity of electric components to be mounted is increased and that allows a manufacture of a wiring board without undergoing a complicated manufacturing process and to provide a multilayer board with an increased reliability, as well as its manufacturing method, that allows miniaturization of a wiring board where electric components are mounted within an insulation layer, and thereby, the quantity of electric components to be mounted is increased; that allows the formation of multiple layers in a collective manner resolving a problem caused by a difference in the thermal histories of the respective layers at the time of formation and allowing for the simplification the manufacturing process; and that allows miniaturization of the wiring board as a result of scale-down and density-increased conductive circuits, achieving an increase in the reliability.